1. Field of the Invention
The invention relates to a composite laser rod, a fabricating method of the composite laser rod, and a laser device that uses the composite laser rod. In particular, the composite laser rod can improve deterioration of the positional stability and output stability of a laser beam that is caused by thermal fluctuation and vibration of a laser rod during laser oscillation, can enhance an absorption efficiency of light that excites the laser rod and thereby can improve the oscillation efficiency, and can enhance a cooling efficiency and thereby can suppress the thermal lens effect.
2. Description of the Related Art
For laser rods that generates laser beam that is employed in welding, boring, repairing, micro-fabrication and so on, crystalline materials are usually used. Among these, single crystals that have garnet structure such as yttrium aluminum garnet (YAG) and so on are mainly used. To the laser rod, a laser active element such as neodymium, ytterbium, thulium, and erbium is doped.
Recently, a laser material that is obtained by doping a laser active element in a transparent material of ceramic YAG that is obtained by baking powder having a YAG composition has been developed and confirmed to have the laser characteristics identical to the single crystal. For instance, Japanese Unexamined Patent Publications (JP-A) H10-67555, H5-235462, H5-286761 and H5-294723 disclose that the transparent ceramic material can be obtained by baking, in a vacuum, powder having a composition of yttrium aluminum garnet (YAG).
Furthermore, in The Review of Laser Engineering, vol. 27, 1999, pp. 593-598, the laser characteristics are reported. Still furthermore, in a YAG single crystal rod, an upper limit of a concentration of Nd that can be introduced is substantially 1.3 atomic %. However, Proceedings (Digest of Technical Papers) of 21st Annual Meeting of The Laser Society of Japan (2001, pp. 40, Lecture No. 30pV3) disclose that the concentration in the ceramic YAG laser rod can be increased to 2% or more. Still furthermore, Y2O3 (yttria) or Sc2O3 that cannot be grown, according to an ordinary crystal growth method, into a crystal excellent in quality and large in size owing to a higher melting point, having thermal conductivity of substantially 20 W/mK that is substantially twice that of YAG, is promised as a laser crystal. When fine and uniform powder of the Y2O3 or Sc2O3 is baked in a vacuum, a transparent and high quality ceramic material can be obtained. It is reported in Proceedings (Digest of Technical Papers) of 22nd Annual Meeting of the Laser Society of Japan (2002, pp. 40, Lecture No. B3-24PI2) that when Nd or Yb is doped in the ceramic material, the ceramic material can obtain laser oscillation.
The laser rod can be excited by use of a flush lamp or a laser diode from a side surface or an end surface, beam emitted therefrom is resonated in a resonator, and thereby a laser oscillation is realized. All energy of the excitation light that is absorbed by the laser active element during the laser oscillation is not converted into energy of laser beam but part thereof is converted into heat. As a result, the laser rod is heated during the laser oscillation and then a temperature is raised. When a temperature of the laser rod varies during the laser oscillation, the refractive index of the laser rod varies. As a result, such problems as that the positional stability of an oscillating laser beam may be deteriorated, and the output strength may fluctuate largely are caused. Accordingly, it is customary to bring the laser rod into close contact with water or a heat sink to cool so that the temperature of the laser rod may be maintained as constant as possible.
Since the laser rod is cooled from a surface thereof, it is inevitable that a temperature gradient in a radial direction is established. When a temperature gradient is generated in the radial direction, since the refractive index also varies according to the temperature, the laser rod exhibits an effect similar to a lens. As a result, light in the rod cannot propagate straight. In order to overcome the thermal lens effect, it is considered to cover a periphery of a single crystal laser rod in which a laser active element is doped with a non-doped single crystal layer. There are proposed several methods for fabricating this composite laser rod. For instance, a method in which a non-doped single crystal layer in which an active element is not doped is disposed around the laser rod of a single crystal in which an active element is doped according to a liquid phase epitaxial growth (LPE) method is disclosed in JP-A-62-140483. Furthermore, a method in which a laser material in which an active element is added and a laser material in which an active element is not added are laminated or thermo-compression bonded is disclosed in U.S. Pat. Nos. 5,441,803 and 5,563,899. Still furthermore, a method in which a hole is bored in a non-doped crystal and a doped crystal to be a core is inserted therein followed by integrating is disclosed in JP-A S63-085152 or JP-A H9-172217.
Recent years, higher precision and higher speed in the laser processing is in demand. For instance, there is a need of forming 1000 holes that has a size of 50 μm in a second at the precision of ±1 μm on a printed wiring board. In order to perform fine processing with high precision in such a short period of time, an improvement in the positional stability and a suppression of the fluctuation of the output strength in a single mode laser beam outputted from a laser oscillator are in demand more than ever.
For the fine processing, since a shorter laser wavelength is more suitable, in many cases, a single mode laser beam is wavelength-converted by use of a wavelength conversion element and used. The wavelength conversion efficiency varies in proportion to a square of an output of the laser beam until the conversion efficiency saturates. Accordingly, when there is a fluctuation in an output of the laser beam of a fundamental wave, the conversion efficiency may vary in proportion to a square of the fluctuation thereof. Furthermore, when an angle of light incident on a non-linear element varies, a light component whose phase matching angle cannot be attained increases. Accordingly, when the positional stability of the beam varies, an output of the wavelength converted light largely varies. From these reasons, in the case of a laser processor that employs the wavelength-converted light, the positional stability of the laser beam that is a fundamental wave has to be improved and the fluctuation of the output strength has to be lowered as large as possible.
One countermeasure to overcome the problems is to maintain a cooling power of cooling water and a heat sink that cool the laser rod at a constant level. However, since when the cooling power is controlled, a temperature at a temperature measurement point is controlled so as to be in a tolerable temperature range, it is impossible to set this temperature range at ±0 degree centigrade. Furthermore, in particular when the cooling water is used, since once elevated water temperature is controlled by returning the cooling water to a chiller, it is very difficult to make completely zero the fluctuation of the water temperature.
Furthermore, there is variation of water pressure when the water is circulated. Accordingly, by devising only a cooling method of the laser rod, required positional stability of the laser beam or output stability thereof can be satisfied with difficulty.
Furthermore, when the laser rod is cooled with the water, there is a problem that a vibration due to a water stream contains a component that matches with a resonant frequency of the laser rod, accordingly the rod begins to vibrate. Still furthermore, also when a heat sink that fixes the rod is air-cooled, the laser rod picks up the vibration due to a cooling fan and so on, as a result, it becomes a factor deteriorating the positional stability of the laser beam and the output stability thereof.
When the laser rod is made larger in its diameter and thereby a volume of the laser rod is increased, a resonant frequency of the laser rod may be lowered, and thereby a problem of the vibration may be overcome. However, when a single mode laser beam that is necessary for fine laser processing is oscillated, a diameter of the laser rod can be made larger only up to substantially 2 mm. Accordingly, the laser rod cannot be made larger up to a diameter that is less influenced by the external vibration due to such as the cooling water or the cooling fan.
Furthermore, it is also a big target to improve the laser oscillation efficiency. In order to facilitate a single mode laser beam to oscillate, it is necessary to concentrate excitation light in the neighborhood of a center of the laser rod. However, in that case, the conversion efficiency from the excitation light to oscillation light becomes such low as substantially 10 to 15%. Accordingly, it is a task to facilitate the laser rod to efficiently absorb the excitation light and thereby to enhance the oscillation efficiency of the single mode laser beam.
Furthermore, when the single mode laser beam is oscillated, since heat addition is concentrated into a slender rod, the thermal lens effect results, as a result, an output laser beam cannot go straight. In order to overcome the problem, as the existing technology, it is considered to dispose a single crystal non-doped layer in the periphery of a single crystal laser rod. However, in the existing technology, it was very difficult to dispose the single crystal non-doped layer to a laser rod having a diameter of 2 mm or less that enables to obtain a single mode.
Accordingly, the invention intends to provide a composite laser rod in periphery of which, a non-doped pipe is bonded, as a structure that can overcome such problems and is less influenced by variation of cooling capacity of cooling water and a heat sink that cool the laser rod and the vibration from a cooling medium. That is, the invention intends to provide a composite laser rod that allows realizing a laser device excellent in the output stability and the beam positional stability, thereby allows improving performance such as processing precision and processing speed of a laser processor, allows improving the oscillation efficiency, and furthermore allows oscillating laser beam excellent in beam quality; a fabricating method thereof; and a laser device therewith.